Neoadjuvant Chemotherapy for Breast Cancer: Evolution of Clinical Practice in a French Cancer Center Over 16 Years and Pathologic Response Rates According to Tumor Subtypes and Clinical Tumor Size: Retrospective Cohort Study

We examined characteristics trends in early breast cancer patients receiving neoadjuvant chemotherapy (NAC) over a 16-year period. Our primary objective was to analyze variations in tumor stage and subtype over time. Secondary objectives included analyses of type of surgery and pathological response, from January 2005 to May 2021, 1623 patients receiving NAC were identified. Three periods were determined: 2005-2009 (P1), 2010-2014 (P2), 2015-2021 (P3). Correlations between periods and patient features with cT stage, pathological breast and axillary node response, pathological complete response (pCR), and type of surgery were assessed in univariate and multivariate analyses. We observed a significant increase in cT0-1 and N0 stages with periods (from 6.8% at P1 to 21.2% at P3, and from 43.2% at P1 to 55.9% at P3, respectively) and in the proportion of HER2+ and triple negative (TN) subtypes. In a multivariate analysis, a decrease of cT2-3-4 tumors during P3 was observed for HER2+ (OR:0.174; p=0.004) and TN tumors (OR:0.287; p=0.042). In-breast pCR and pCR were observed in 40.8% and 34.4% of all patients, respectively, with strong association with tumor subtypes, but not with tumor size in multivariate analysis (37.0% pCR for cT0-1 tumors, 36.4% for cT2 tumors, 29.1% for cT3 tumors (cT0-1 versus cT≥2; p=0.222)). pCR was negatively associated with cN1 stage (OR:1.499; p<0.001 for cN1 patients compared to cN0). We observed an increase in the proportion of small cT0-1 and N0 stages treated with NAC, especially in HER2+ and TN subtypes. No significant impact of tumor size on pCR rates was found.


Introduction
Neoadjuvant chemotherapy (NAC), as defined as the administration of cytotoxic treatment before tumor surgical removal, has been implemented in breast cancer (BC) during the last 50 years, encompassing different objectives. Initially dedicated to inoperable BC to convert it into a surgically removable disease [1,2], NAC was then widely developed in operable BC when frontline breast-conserving surgery with "in sano" margins was not feasible [3], based on accumulating data showing a high rate of clinical and pathologic response (pCR) as well as identical survival outcome when compared to adjuvant chemotherapy [4]. Yet, the development of more and more sophisticated oncoplastic techniques could also increase the possibilities of immediate breast-conserving surgery [5,6]. Even though some initial expectations in favor of NAC were not confirmed, including a potential survival advantage over adjuvant chemotherapy by an early impact on the hypothetic surgical stimulation of micro-metastatic disease and tumor shedding [7], the interest in NAC has been further boosted during the last decade in relation with several observations. First, molecular analysis of BC revealed that the disease was composed of different subtypes with distinct survival outcomes [8,9], including HER2-positive and triple-negative BC, two subtypes in which both survival benefits from adjuvant chemotherapy and probability of reaching pCR after NAC are the highest. Second, a tight relationship was demonstrated between the achievement of pCR and survival, most notably in the latter subtypes [10][11][12]. Third, recent randomized trials found that in those patients with residual invasive disease after preoperative systemic treatment, adjuvant trastuzumab emtansine in HER2-positive BC treated with trastuzumab-based NAC [13] and capecitabine in triple negative (TN) BC treated with anthracycline-taxane NAC [14] both significantly improved survival. These recent practice-changing results have resulted in making NAC a standard of care in most of these BC subtypes, even when initial breast conservation may be achieved by frontline surgery. In this retrospective monocentric study, we have examined how the use of NAC has evolved during the last 16 years in a French comprehensive cancer center, including clinical and pathological features of treated patients, as well as pathologic complete response (pCR) results. The primary objective was to analyze variations in tumor stage and subtype over time. Secondary objectives included analyses of the type of breast and axillary surgery as well as pathological axillary and in-breast response.

Patient selection and study design
Medical records of early BC patients treated from January 2005 to May 2021 were retrieved from our institutional clinical databases for retrospective analysis. This cohort study was approved by our institutional review board (Registered study in Clinical research Institute: NAC-TS-IPC 2021-026). Patient and tumor characteristics, periods, treatments, and pathological results were collected. We included in the present study 1623 patients treated with NAC, without metastasis at initial diagnosis (figure 1). Patients receiving NAC were staged using clinical examination, mammography and ultrasonogaphy, breast MRI. Search for distant metastases using either PET-scan or a combination of CT-scan and bone scan. Evaluation of lymph node status was determined by sentinel lymph node biopsy (SLNB) with or without completion axillary lymph node dissection (ALND) or only ALND. SLNB was realized before NAC for cN0 patients with cT ≤ 5 centimeters. The method used for the detection of SN was a combined technique or isotopic only detection during the last years. NAC included anthracyclines plus taxanes-based regimen. All patients with HER2+ disease received trastuzumab during NAC. Patients who had residual components of ductal carcinoma in situ were assessed as having a breast pathological complete response based on the National Surgical Adjuvant Breast and Bowel Project criteria [15]. Patients with axillary residual lymph node tumor (ypN1) were assessed as having no pathological complete response. Pathological complete response (pCR) was defined as [ypT0 or ypTis] and [ypN0 or pN0sn or pN1sn without ALND or ypNx (absence of axillary surgery)] [16]. Endocrine Receptor (ER) and HER2 status were determined according to French guidelines (estrogen and/or progesterone receptors by IHC with a 10% threshold for endocrine receptor (ER) positivity; IHC HER2-positivity score of 3+ and/or HER2 amplification by in situ hybridization) [17,18]. Five IHC tumor subtypes were defined as surrogates for molecular subtypes based on tumor grade, ER and HER2 status: luminal A-like (ER+/HER2−/grade1 or 2), luminal B-like (ER+/HER2−/grade 3), luminal B-like HER2+, HER2+ (ER−/HER2+), and triple-negative (ER−/HER2−) [16]. The work has been reported in line with the STROCSS criteria [17].

Statistical Analysis
Descriptive statistics were used to describe the categorical (counts and frequencies) and continuous (median and range) variables. Characteristics of patients were compared by different periods: Period 1 (P1) 2005-2009, period 2 (P2) 2010-2014, and period 3 (P3) 2015-2021 by using χ2 test for categorical variables, and Kruskal-Wallis test for continuous variables. The main characteristics of patients and tumors were categorized (cT0-1 stage vs. cT2-3-4, ypT0-is vs. ypT≥1, pCR vs. no pCR, breast conservative surgery vs. mastectomy) and association with other variables were explored in univariate, and multivariate analysis by binary logistic regression for significant criteria in univariate analysis. Statistical significance was set as p ≤ 0.05. Analyses were performed with SPSS version 16.0 (SPSS Inc., Chicago, Illinois).

Characteristics of patients according to periods
Of 1629 patients included, 280 received NAC during P1, 503 during P2, and 846 during P3. Median age for all patients was 50.0 years and was stable among periods. Characteristics of patients are reported in table 1. All the other assessed criteria were significantly associated with periods of treatment. Notably, we observed a significant increase in the proportion of cT0-1 (6.8% at P1 to 21.2% at P3), and node-negative tumors ( 28.6%, and 37.5%) increased. Of note, the rate of residual invasive disease in breast and nodes also significantly decreased across the three periods (Table 1).

Axillary surgery details for all patients and for clinically N0 patients
For all patients, axillary surgery type (SLNB, ALND, or SLNB+ALND) was significantly associated with periods, tumor subtypes, cT, and cN stages (

cT Stages according to periods
cT stage (cT0-1 versus cT2-3-4) was significantly associated with periods, and cN stages, while the proportion of different subtypes did not differ significantly by univariate analysis ( Table 4). The multivariate analysis revealed a strong association of cT2-3-4 tumors with cN≥1 (OR: 1.748; p<0.0001), and a notable decrease of cT2-3-4 stages during the third period (OR: 0.270 ; p<0.0001) (

Discussion
In this large single-center retrospective study covering a broad 16-year period, we reported an increase in NAC use in successive years for small (cT0-1) tumors, and an increase in the use of SLNB before NAC with no significant difference in pCR and mastectomy rates. In locally advanced and large breast cancers, particularly when mastectomy is required, NAC is recommended to decrease mastectomy rate [19]. In operable patients, NAC has not demonstrated impact on survival in comparison with adjuvant chemotherapy after breast surgery [7,19]. However, NAC allows assessment of pathological response, which has prognostic value, most notably in HER2+ and triple-negative subtypes and can guide the indication of postoperative treatments with a significant reduction in the risk of recurrence, as demonstrated with trastuzumab emtansine and capecitabine in these subtypes, respectively. Consequently, NAC should be preferred for triple-negative and HER2-positive tumors > 2cm [19]. However, NAC could also be discussed for smaller triple-negative and HER2-positive, cT1c, and sometimes cT1b tumors, especially for cN1 or pN1sn patients when SLNB is performed before NAC. Indeed, in HER2+ BC, trastuzumab-based adjuvant chemotherapy has demonstrated robust survival benefits in randomized trials in patients with pT1c stage or higher [20][21][22][23]. In addition, accumulating data suggest similar benefits even in sub-centimetric tumors, more convincingly in pT1bN0 than pT1aN0 tumors [24]. Of note, the KATHERINE study [13], in which post-operative trastuzumab emtansine was administered in the presence of invasive residual disease after trastuzumab-based NAC, demonstrated a significant improvement in invasive recurrence, 12% of patients had T1 tumors and there was no evidence in favor of a lower benefit compare to higher tumor stage (HR=0. 33  , and T3, respectively). Thus, since trastuzumab-based chemotherapy provides survival advantages and is commonly recommended even in small tumors, and since the absence of pCR may allow an effective rescue, it is tempting to consider a novel paradigm in which virtually any HER2+ tumors > 5mm may be offered NAC. However, during the same period, a relative de-escalation was proposed to T1-T2 (up to 3cm) node-negative HER2-positive BC patients. Thus, in this patient population, a shortduration (12 weeks) anthracycline-and alkylating-free regimen, using weekly paclitaxel in combination with trastuzumab may be proposed with minimal toxicity [25]. Whether such a regimen could similarly be used in the neoadjuvant setting with a trastuzumab emtansine-based rescue strategy warrants further investigation. Similarly, in triple-negative BC patients, adjuvant chemotherapy is largely recommended whatever tumor size (NCCN, ESMO, St-Gallen), even though survival benefit in pT1abN0 remains unclear [26,27]. Once again, in this subtype, adjuvant capecitabine, in patients receiving neoadjuvant anthracycline +/− taxane-based chemotherapy and with the persistence of residual invasive cancer on surgical specimen has been demonstrated to improve survival, according to CREATE-X phase III trial [14].  [13,[20][21][22][23][28][29][30][31][32][33][34][35]. However, subgroup analyses of some individual studies [28] and recently reported meta-analyses have suggested significant improvements in triple-negative BC patients, most of benefits being driven by studies in which NAC was used to select patients with the highest risk [36,37]. Accordingly, adjuvant capecitabine in patients not reaching pCR after NAC has become standard of care in this subtype. Thus, neoadjuvant anthracycline-taxane chemotherapy could be proposed to any pT1b-c triple-negative breast cancer to better define the need for additional cytotoxic such as capecitabine. Recent results of the OLYMPIA trial [38], in which olaparib given to patients with germline BRCA mutation and BC with invasive residual disease (any invasive in triple-negative BC, high CPS+EG score in ER+ BC) after neoadjuvant treatment, was shown to improve significantly survival may further support this approach in this specific subgroup of patients. The rate of conservative surgery appears to be stable over time, with higher rates for triple-negative tumors. However, the rate of conservative surgery depends on many factors, including the initial tumor size, as we have observed for cT3 and cT4 tumors, and on the patient's choice of whether or not to have a total mastectomy in the presence of hereditary risk or mutation, when conservative surgery is feasible. This choice may also have evolved with the increased possibilities and proposals for immediate reconstruction. However, patients must be informed of the aesthetic limitations of immediate or delayed reconstruction [39], postoperative complications [40], and of the risks of altering the aesthetic results in the event of post-mastectomy radiotherapy.
As expected, pCR rates following NAC were significantly different according to tumor subtypes, being higher in HER2+ and triple-negative BC. Logically, the use of NAC is currently predominantly dedicated to these subtypes. Importantly, no significant differences in pCR according to tumor size was observed, which also argues in favor of considering NAC even in smaller tumors, as discussed above. There was a trend for a slightly higher pCR in the last period, which did not reach statistical significance. This may be due to the lack of significant change in NAC regimen during the considered periods, most HER2+ patients having access to trastuzumab, while pertuzumab or platinum were not routinely used in France in HER2+ and triple-negative subtypes, respectively. Axillary exploration and treatment have evolved significantly in recent years. The performance of an SLNB for cN0 patients (clinical and ultrasound) can be considered before or after NAC [41][42][43]. SLNB prior NAC for tumors ≤5cm in patients without clinical axillary invasion (cN0) is performed under the same conditions as surgeries performed at the outset. This approach has been proposed for many years [44] and validated more recently by the SENTINA study [41]. In these situations, when the sentinel lymph nodes are not involved, the omission of an additional ALND has been recommended since the results of the NSABP B-32 trial, which validated the sentinel lymph node technique without dissection for immediate surgery [45], and since the results of the SENTINA trial, which included an axillary dissection prior to NAC in one of the randomized arms [41]. In case of sentinel lymph node involvement, completion of ALND after NAC is the attitude that has been usually advocated. However, since the results of the ACOSOG Z0011 and IBCSG 23-01 trials, the omission of completion ALND after NAC can be considered as it is the case for immediate BC surgery, in line with the criteria corresponding to the situations of these trials: presence of at most two macro-or mi-cro-metastases in the sentinel lymph nodes without macroscopic capsular rupture with the realization of breast-conserving surgery, whole-breast radiotherapy, and systemic treatment. Under these conditions, the number of cases with more than 2 sentinel nodes affected being low, an omission of completion ALND would be possible for the vast majority of patients. The presence of micro-metastases or isolated cells is considered to have a prognosis equivalent to pN0sn [19] in case of ER+ tumor [46] but with an unfavorable prognostic value for triple-negative cancers [46]. The disadvantage of this approach is the need for additional surgery, which should not delay the initiation of NAC. On the other hand, the advantage is to precisely know the lymph node stage, in particular to evaluate the indication of regional lymph node radiotherapy, at least in case of macro-metastasis of the sentinel lymph nodes. However, a significant limitation of omitting ALND after initial positive SLND includes the lack of nodal staging allowing to certify pCR status according to the most consensual definition. Yet, in HER2+ and triple-negative BC, a residual nodal involvement when breast pCR is achieved is expected to be uncommon (9.6 to 18.4% in our study). However, the most common approach proposed for cN0 patients, both clinically and ultrasonographically, is to perform an SLNB after NAC [19]. The downstaging linked to the sterilization of initially affected lymph nodes allows considering the omission of an additional ALND in a significant proportion of patients. Conversely, in the case of an invaded node after NAC, a factor associated with poor prognosis, additional ALND is indicated for micro or macro-metastases. The advantage of this approach is to determine the pathologic response of the lymph nodes to NAC. Another advantage is to offer ALND only to a small number of patients, which is probably not higher than the rate of abstention from dissection if SLNB is performed before NAC and without dissection for invaded pN0sn and <2 SN. The disadvantage is that the initial lymph node status is not known, which may be the only element determining the indication or not of regional lymph node radiotherapy. In patients with initial limited axillary lymph node involvement confirmed on needle aspiration or biopsy (cN1), SLNB may be considered in selected cases with clinical response in the axillary region, based on the results of the SENTINA and ACOSOG Z1071 trials [41,48]. However, the rate of false negatives remains high overall, from 8% to 14.2 [41,48,49]. This false-negative rate can be reduced by marking the biopsied lymph node (clip or other technique) and by sampling more than 2 sentinel lymph nodes whenever this number is reached by identification [50,51]. Although our study includes a considerable number of early BC patients receiving NAC on a broad 16-year period, limitations must be addressed. First, potential selection bias and lack of standardization in treatment strategies inherent in the retrospective design of the study. Second, we chose to include patients until May 2021 and thus, assessment of independent causality between pathologic response and outcome was not possible. Third, no specialized pathology review of all cases was undertaken.

Conclusions
The pCR rate after NAC was neither higher nor significantly different for tumors <2cm compared with tumors ≥2cm, especially for HER2+ and TN subtypes for which adjuvant therapy can be offered to improve prognosis. The pCR rate appears to correlate with intrinsic tumor characteristics and clinical lymph node status rather than with tumor size. These results suggest that it is possible to propose NAC in patients with these tumor subtypes, in case of clinically invasive axillary lymph nodes (cN1) but also in the absence of suspicious lymph nodes (cN0) when the tumor is smaller than 2cm. An evaluation of overall survival and recurrence-free survival comparing patients with these sizes of tumors and adjusted for other prognostic factors should be performed to confirm these NAC indications.

Data availability statement
Not applicable.